1. Field of the Invention
The present invention relates to a method for treatment of tissue, for example, collagenous tissue, where a deleterious mechanical loading environment contributes to the degradation of the tissue.
2. Description of the Related Art
Deleterious mechanical loading environments contribute to the degradation of collagenous tissue in a variety of manners. For instance, fatigue is a weakening of a material due to repetitive applied stress. Fatigue failure is simply a failure where repetitive stresses have weakened a material such that it fails below the original ultimate stress level. Elevated stress levels, due to tissue removal, can accelerate fatigue degradation of the remaining joint tissues. In bone and other diarthrodial joint tissues, two processes—biological repair and fatigue—are in opposition, and repair generally dominates. In the intervertebral disc, the prevalence of mechanical degradation of the posterior annulus (Osti 1992) suggests that fatigue is the dominant process. The intervertebral disc, being the largest, principally avascular load supporting tissue in the body, is somewhat unique in this predisposition toward ongoing fatigue degradation. Active tissue response (adaptation, repair) does not play a strong role in the case of mature intervertebral disc annular material. The intervertebral disc is comprised of three parts: the nucleus pulposus (NP) or nucleus, the annulus fibrosus (AF) or annulus, and the cartilaginous endplates. The characteristic of the inner annulus and outer nucleus blend with ongoing degeneration, with the nucleus becoming more fibrous and decreasing in water content. Similarly, the boundary between outer nucleus and inner annulus is known to fade and becomes indistinct with ongoing degeneration. As a principally avascular structure, the disc relies on diffusion and loading induced convection for nutrition of its limited number of viable cells. Age related changes interfere with diffusion presumably contributing to declining cell viability and biosynthetic function (Buckwalter et al. 1993, Buckwalter 1995). Age related decline in numbers of cells and cell functionality compromises the ability of the cells to repair mechanical damage to the matrix. Some regeneration of the matrix in the nucleus following enzymatic degradation has been accomplished, albeit inconsistently (Deutman 1992). Regeneration of functional annular material has not yet been realized.
Combined with this limited potential for repair or regeneration, studies have shown that posterior intervertebral disc tissue is vulnerable to degradation and fatigue failure when subjected to non-traumatic, physiologic cyclic loads. Prior work has shown deterioration in elastic-plastic (Hedman 99) and viscoelastic (Hedman 00) material properties in posterior intervertebral disc tissue subjected to moderate physiological cyclic loading. Cyclic load magnitudes of 30% of ultimate tensile strength produced significant deterioration of material properties with as little as 2000 cycles. Green (1993) investigated the ultimate tensile strength and fatigue life of matched pairs of outer annulus specimens. They found that fatigue failure could occur in less than 10,000 cycles when the vertical tensile cyclic peak exceeded 45% of the ultimate tensile stress of the matched pair control. In addition, Panjabi et al (1996) found that single cycle sub-failure strains to anterior cruciate ligaments of the knee alter the elastic characteristics (load-deformation) of the ligament. Osti (1992) found that annular tears and fissures were predominantly found in the posterolateral regions of the discs. Adams (1982) demonstrated the propensity of slightly degenerated discs to prolapse posteriorly when hyperflexed and showed that fatigue failure might occur in lumbar discs as the outer posterior annulus is overstretched in the vertical direction while severely loaded in flexion. In an analytical study, interlaminar shear stresses, which can produce delaminations, have been found to be highest in the posterolateral regions of the disc (Goel 1995). These prior data indicate: 1) the posterior disc and posterior longitudinal ligament are at risk of degenerative changes, and that 2) the mechanism of degeneration can involve flexion fatigue.
A different type of mechanical degradation of collagenous tissue occurs in scoliosis and other progressive spinal deformities. Scoliosis refers to an abnormal lateral, primarily, or other curvature or deformity of the spine, often of unknown origin. Progressive spinal deformities can also occur subsequent to surgical bone removal, with or without accompanying spinal instrumentation, such as in a neural decompression procedure or subsequent to vertebral failure. The bony vertebral failure itself may occur as a result of trauma or of age related osteoporosis or osteopenia. Kyphotic deformity (loss of outward concavity or increase in outward convexity), in the lumbar spine also known as flat-back syndrome, is a frequent sequela to spinal fusion or installation of spinal instrumentation, especially in the case of a long, multi-level, surgical construct. Severe curvature and ongoing curve progression can lead to many other health disorders including but not limited to compromised respiratory function. In addition, one's lifestyle can be adversely affected and a loss of cosmesis can result. A large segment of the population is affected by scoliosis, approximately 2% of women and 0.5% of men. Over 80% of scoliosis is of no known origin (i.e., idiopathic). Approximately 80% of idiopathic scoliosis develops in young pubescent adults. The incidence of deformity increases with age. Existing conservative approaches to limit curve progression such as external bracing can be awkward or restricting, and are of disputed value. Surgical correction of severe curves can be intensive with a long recovery period, require the need for post-operative bracing, and be fraught with many other post-operative problems.
Another form of spinal deformity, spondylolisthesis commonly occurs in the lower lumbar region of the spine. Spondylolisthesis involves the slippage of one vertebral level relative to an adjacent level. Progressive listhesis leads to sciatica and pain. Surgical intervention is an option to prevent progressive slip, especially when the slip has reached a greater amount of slip displacement or slip angle. However, nonsurgical means of preventing a slip to progress to the point where surgery is indicated have not been available previously.
Current treatments for scoliosis and other progressive spinal deformities consist of bracing and surgery. The purpose of orthopedic braces is to prevent increasing spinal deformity, not to correct existing deformity. Braces are generally used in children with an expected amount of skeletal growth remaining, who have curve magnitudes in the range of 25 to 40 degrees. External braces are routinely used as a standard of care. Yet there is controversy regarding the effectiveness of external bracing. The magnitude of forces delivered to the spine corresponding to brace loads applied to the torso cannot be quantified directly. Larger forces applied to the torso may also result in brace induced pathologies to the tissues in contact with the brace. Some studies suggest that braces are effective in halting curve progression in about 80 percent of afflicted children. But because the option to do nothing but observe curve progression is inappropriate, there is no generally accepted percentage of these curves that would stop progressing on their own or due to other factors.
Naturally occurring collagen crosslinks play an important role in stabilizing collagenous tissues and, in particular, the intervertebral disc. Significantly higher quantities of reducible (newly formed) crosslinks have been found on the convex sides than on the concave sides of scoliotic discs (Duance, et al. 1998). Similarly, Greve, et al. (1988) found a statistically increased amount of reducible crosslinks in scoliotic chicken discs at the same time that curvatures were increasing. This suggests that there is some form of natural, cell-mediated crosslink augmentation that occurs in response to the elevated tensile environment on the convex side of scoliotic discs. Greve also found that there were fewer reducible crosslinks at the very early stages of development in the cartilage of scoliotic chickens. They concluded that differences in collagen crosslinking did not appear to be causative because there was not a smaller number of crosslinks at later stages of development. In fact, later on, when the scoliotic curve was progressing, there were statistically significant greater numbers of collagen crosslinks, perhaps in response to the curvature. Although not the conclusion of Greve, this can be interpreted as being a sufficient depletion of crosslinks in the developmental process with long enough duration to trigger the progression of scoliotic curvature that was later mended by a cellular response that produced higher than normal levels of crosslinks. These studies suggest that the presence of collagen crosslink augmentation mechanisms may be critical to prevent ongoing degradation and for mechanical stability of intervertebral disc tissue in scoliotic spines and when tensile stresses are elevated.
It is important to note that these studies did not quantify the integrity or crosslink quantities associated with the elastin and elastic fiber network which also plays a role in the mechanical integrity of these collagenous materials. Some of the benefit of crosslinking of the principally collagenous tissues like the intervertebral disc may also be attributed to an effect on the elastin and elastic fiber network and other proteins (such as link proteins) in these collagenous tissues. In the same way that intramolecular, intermolecular and interfibrillar crosslinks of collagen molecules and fibers benefit the tissue and joint mechanics, including resistance to degradation, tears and deformity, and increased permeability, intramolecular, intermolecular and interfibrillar crosslinks involving elastin and the elastic fiber network could provide benefits to the tissue and joint mechanics and nutrition. In fact, the same reagents effective at augmenting collagen crosslinking may also augment crosslinks involving the elastin and elastic fiber network, or other tissue proteins.
It is well documented that endogenous (naturally occurring—enzymatically derived and age increasing non-enzymatic) and exogenous collagen crosslinks (historically applied to implants) increase the strength and stiffness of collagenous, load-supporting tissues (, Chachra 1996, Wang 1998, Sung 1999a, Zeeman 1999, Chen 2001). Sung (1999b) found that a naturally occurring crosslinking agent, genipin, provided greater ultimate tensile strength and toughness when compared with other crosslinking reagents. Genipin also demonstrated significantly less cytotoxicity compared to other more commonly used crosslinking agents. With regard to viscoelastic properties, Lee (1989) found that aldehyde fixation reduced stress-relaxation and creep in bovine pericardium. Recently, naturally occurring collagen crosslinks were described as providing ‘sacrificial bonds’ that both protect tissue and dissipate energy (Thompson, et al. 2001). To date, there is no known reference in the literature as to the ability of exogenous crosslinks to decrease the viscoelastic characteristic of hysteresis or to increase the ability of the collagenous tissue to store energy. A need therefore exists to find biochemical methods that enhance the body's own efforts to stabilize discs in scoliotic and other progressively deforming spines by increasing collagen crosslinks.
Mechanical degradation of collagenous tissue can also occur if the environment for biological activity in the central region of the disc is poor. Tissue engineering is a burgeoning field which aims to utilize cells, special proteins called cytokines and synthetic and native matrices or scaffolds in the repair and regeneration of degraded, injured or otherwise failed tissues. With regard to the intervertebral disc, biological solutions like tissue engineering are hindered by the harsh, hypoxic (oxygen deficient) avascular (very little if any direct blood supply) environment of moderately degenerated intervertebral discs. The disc is known to receive nutrients and discard cell waste products primarily by diurnal-cyclic pressure driven fluid flow and diffusion through the annulus fibrosus and through the cartilaginous endplates that connect the disc to the bony, well vascularized, spinal vertebrae. The disc cartilaginous endplates lose permeability by calcification while the disc itself becomes clogged up with an accumulation of degraded matrix molecules and cell waste products. This loss of disc permeability effectively reduces the flow of nutrients to the cells and the flow of waste products from the cells in the interior central region of the disc, the nucleus pulposus. This loss of flow of nutrition to the disc causes a loss of cell functionality, cell senescence, and causes a fall in pH levels that further compromises cell function and may cause cell death (Buckwalter 1995, Horner and Urban 2001). Horner and Urban showed that density of viable cells was regulated by nutritional constraints such that a decline in glucose supply led to a decrease in viable cells. Boyd-White and Williams (1996) showed that crosslinking of basement membranes increased permeability of the membranes to macromolecules such as serum albumin, crosslinked albumin, and a series of fluorescein isothiocyanate dextrans of four different molecular sizes. It is herein suggested, then, that increased crosslinking of the annulus fibrosus and/or the endplates of intervertebral discs, though very different and more complex collagenous tissues than basement membranes, would provide for increased flow of glucose and other nutritional macromolecules to cells and waste products from the cells in the interior region of the disc, thus improving their viability.
Intervertebral disc herniation involves fissuring, rupture or tearing of the annulus fibrosus followed by displacement of the central portion of the disc posteriorly or posterolaterally through the torn tissue. The deformed or displaced disc protrusion can compress a nerve root and/or the spinal cord. Clinical symptoms associated with herniated disc include back pain and radiculopathy including leg pain, sciatica and muscle weakness. Treatments for herniated disc commonly comprise excision of the protruding disc segment and other tissues suspected to be involved with nerve compression and pain. Prior to tearing through the outer annular fibers the disc can bulge posteriorly potentially applying pressure to neural elements. Approximately a decade typically separates the first, acute incidence of low back pain and the onset of radicular symptoms. There is currently no treatment available to prevent degeneration, annular tearing, nucleus migration, herniation and sciatica.
Similarly, emerging nucleus augmentation or replacement technologies rely on the integrity of a surgically weakened annulus fibrosus to prevent migration and extrusion or expulsion of implanted materials or devices. These materials and devices are typically targeted for patients in the early stages of disc degeneration (Galante I-III), where there is less degradation of the annulus fibrosus because of the reliance on annulus integrity for the success of these implants. However the annulus is typically compromised further in order to implant these materials and devices to the central region of the disc. Clinical data at this time suggests that implant migration and extrusion is one of the main complications to this type of treatment. High rates of extrusion have been reported for some nucleus replacements, 10% for the device with the most clinical experience and 20-33% for another. A need therefore exists for a method for resisting annulus tearing after or at the same time of implantation of a nucleus augmentation or nucleus replacement devices. Normal physiological loading can displace, extrude or expulse devices and materials implanted into the center, nucleus region of the intervertebral disc. Consequently, a treatment capable of improving annulus tear resistance could be useful both to prevent eminent disc protrusions and as an adjunct to a disc augmentation or nucleus replacement procedure.
To date, no treatments capable of reducing mechanical degradation to native, non-denatured collagenous tissues currently exist. In fact, no other collagenous tissue fatigue inhibitors have been proposed. A need therefore exists for a method for improving the resistance of collagenous tissues in the human body to fatigue and for otherwise reducing the mechanical degradation of human collagenous tissues, in particular, the posterior annulus region of the intervertebral disc. In addition, a need exists to increase resistance to scoliotic curve progression and other progressive spinal deformities by treatment of appropriate regions on the tensile side (convex) of affected discs and to improve permeability, particularly the hydraulic and macromolecular permeability and diffusivity of the outer region of the disc, but also throughout the disc annulus in whole or in part and the cartilaginous endplates of the disc, the flow of nutrition, such as glucose and other nutritional macromolecules, to cells in the annulus and in the central portion of the disc, and the flow of waste products from the cells.
Spinal deformities following vertebral fractures including kyphotic deformities following vertebral compression fractures are sometimes treated by injecting a cement-like material into the intravertebral space (vertebroplasty) sometimes following a vertebral height restoration procedure to reduce the deformity (kyphoplasty). A complementary procedure to increase the tension band restraint by increasing and improving elastic characteristics of the tensile side of the affected discs would also be beneficial in preventing the incidence of deformity as well as the progression of the deformity. Improvement of the tension band characteristics in this way stabilizes the spinal column and is a means of internal, natural bracing.